The aluminizing process is well-known to the art for improving the corrosion resistance of many substrates such as alloys containing chromium, iron, nickel or cobalt, for example, as the major constituent and in particular, improving the properties of the nickel-and cobalt-based superalloys which are used in high-temperature environment, such as gas turbine blades and vanes. In a typical aluminizing process, the part to be aluminized would be embedded in a pack powder containing aluminum, either as the metal or alloyed with another metal such as cobalt, for example; a carrier, typically an ammonium or alkali metal halide, and an inert filler such as alumina. The part would be embedded in this pack and heated to 1200.degree. F.-2200.degree. F. to coat the external surface of the part with aluminum; the thickness of the coating depending upon the time and temperature of treatment. In this process, the halide acts as a carrier or activator to facilitate the transfer of the aluminum from the source of the aluminum to the part. Representative patents showing these types of pack cementation processes are those to Puyear et al., U.S. Pat. No. 3,079,276 dated Feb. 26, 1963, for Vapor Diffusion Coating Process; Fitzer, U.S. Pat. No. 2,886,469 dated May 12, 1959, for Method of Coating Metallic Bodies with Aluminum Utilizing Vaporous Sub-Chlorides; Zupan, U.S. Pat. No. 3,335,028 dated Aug. 8, 1967, for Complex Fluoro Salt Cementation Method for Coating Refractory Metallic Substrates; Brill-Edwards U.S. Pat. No. 3,694,255 dated Sept. 26, 1972, for Method For Coating Heat Resistant Alloys and Speirs et al. U.S. Pat. No. 3,764,373 dated Oct. 9, 1973, for Diffusion Coating of Metals.
When internal passages are required to be coated using the pack process, difficulties are encountered because the internal passages, which in many cases are quite small, must be filled with the pack powder mixture and after treatment, these internal passages must be cleared of any residual pack powder mixture. In addition, coating deposition is effected by the ratio of pack powder to surface area to be coated. Therefore, internal part geometry effects the thickness of coating deposited. Thus, when internal passages are to be coated, attempts have been made to use a gas phase deposition process in which the part would be suspended outside of the powder mixture and a gaseous, aluminum-containing material caused to flow through the part. A typical approach is, for example, described in the patent to Gauje U.S. Pat. No. 3,486,927 dated Dec. 30, 1969, for Process For Depositing A Protective Aluminum Coating on Metal Articles. It should be noted that the Gauje patent was not directed specifically to the coating of internal surfaces and when attempts were made to adapt the Gauje process to the coating of internal passages, it was not possible to obtain a usably uniform internal coating of aluminum.
According to this invention, we have found that the use of complex aluminum halides of an alkali metal or an alkaline earth metal as the activator substantially increases the "throwing power" of a gas phase deposition process for the aluminizing of internal surfaces and a usably uniform coating is obtained. Also, the effect of part internal geometry on coating thickness is greatly reduced.
It is, accordingly, an object of this invention to provide gas phase aluminizing process particularly adapted for the coating of internal passages.
This and other objects of the invention will be readily apparent from the following description.